Drought is a common abiotic stress that limits the productivity of all major crops. About 80% of the agricultural land in the United States experienced drought in 2012, impacting 70-75% of corn and soybean acreage (United States Department of Agriculture, Economic Research Service, 2012). According to the United Nations, drought intensity is increasing worldwide (United Nations News Center, 2012). Even seasonal mild or moderate drought in critical growth stages can reduce yields by 10-20% on rain-fed farms or those with limited irrigation. According to the National Climatic Data Center, both moderate and severe to extreme drought is becoming more common. Coupled with the increasing depletion of water resources, there is a need for new products and solutions to meet drought stress. Several approaches are being used with varying levels of success to address this problem including agronomics, traditional plant breeding, genetic engineering and chemical treatments. Each of these strategies has potential benefits, but also significant shortcomings.
There are a number of cultural practices in crop production designed to help avoid drought effects. A “drought escape” strategy can be employed by planting suitable varieties early in the season so they mature before the onset of late summer drought. A “drought avoidance” strategy can be used by selecting plant varieties with a deep root system, reduced leaf area and/or rapid stomatal closure. These strategies may have undesirable consequences. Drought escape involves a shortened or shifted growing season, while drought avoidance mechanisms may divert carbon into non-harvestable sinks Plant breeders incorporate drought tolerance traits into crops as a part of regular yield improvement programs. However, the process of breeding is slow and labor-intensive even when assisted by molecular markers. Recent breeding efforts by the seed industry have resulted in elite hybrids with 10-15% greater yield under moderate drought.
Genetic engineering offers precise tools to alter plant traits. Since the late 1990s, transgenic expression of “drought tolerance” genes has been pursued as a method of boosting crop performance under drought. For example, Monsanto Company's Genuity° DroughtGard™ Hybrid corn was approved by the United Stated Department of Agriculture and United Stated Environment Protection Agency for commercial cultivation. This hybrid corn has demonstrated a ˜6% yield increase under moderate drought.
Chemicals that have been promoted and used commercially to alleviate the effects of drought include abscisic acid, anti-transpirants, and triazole growth inhibitors (e.g. uniconazole). For example, anti-transpirants reduce gas exchange and thus inhibit water loss. However, reduction of gas exchange inhibits photosynthesis, and thus slows plant growth. Although these chemicals may be effective at combating drought, they may not be acceptable for use in field crops due to negative effects on yield, cost, adverse side effects, or short duration of effect.
Accordingly, there is a need for new methods to improve cereal grain response to drought stress and to improve yield of cereal grains.